ABSTRACT Measurement of cortisol in hair is a potentially useful form of measurement that is rapidly becoming popular, especially in studies involving babies and children, This is because it confers many benefits that are not found with traditional forms of measurement. These include being non-invasive, easy to collect and the ability of one small specimen to provide retrospective information about the time that the hair was growing. However, there is currently a lack of biological validation to understand how to correctly interpret hair cortisol data. This proposal aims to conduct systematic studies of hair cortisol and its metabolites (hair glucocorticoids) using a rhesus monkey model. Rhesus monkeys are the ideal model for this study as their endocrinology is similar to humans, and therefore, results can directly be translated to humans. Completion of this proposal will move forward the ability to use and understand hair glucocorticoid data in three ways. First, it will determine the type of study for which it is appropriate to utilize hair glucocorticoids. Using a pharmacological approach, it will be shown whether acute and/or chronic changes in circulating cortisol are reflected in the hair. This knowledge will allow for ensuring that the study design is appropriate for type of stress that is being evaluated and will provide the knowledge by which hair glucocorticoid data can be interpreted. Second, of the role of cortisol production in hair follicles as a mechanism by which cortisol is accumulated in the hair shaft will be elucidated. In order to correctly interpret hair glucocorticoid data, it is critical to understand the mechanism by which the hormone was incorporated into the hair. In this proposal, it will be determined if local production of cortisol in the hair follicle is a viable mechanism and, if so, the time- course and nature of disposition of the hormone. Finally, all of the experiments in this proposal will be conducted with the knowledge that hair cortisol is not the only glucocorticoid metabolite that is important in hair. All of the measurements will be conducted with liquid chromatography-tandem mass spectrometry so there will be simultaneous assessment of hair cortisol, hair cortisone and other metabolites to determine which glucocorticoids or ratio of metabolites provide the best measurement and under what conditions. Overall, fulfillment of the aims of this proposal will permit interpretation of hair glucocorticoids in a valid, reliable and biologically appropriate manner.